In his unpublished manuscripts (referred to by Birch [1] as Fragment V, pp. 247–249), Ramanujan [3] gave a whole list of assertions about various (transforms of) modular forms possessing naturally associated Euler products, in more or less the spirit of his extremely beautiful paper entitled “On certain arithmetical functions” (in Trans. Camb. Phil. Soc. 22 (1916)). It is simply amazing how Ramanujan could write down (with an ostensibly profound insight) a basis of eigenfunctions (of Hecke operators) whose associated Dirichlet series have Euler products, anticipating by two decades the famous work of Hecke and Petersson. That he had further realized, in the event of a modular form f not corresponding to an Euler product, the possibility of restoring the Euler product property to a suitable linear combination of modular forms of the same type as f, is evidently fantastic.
Raghavan, S. On Ramanujan and Dirichlet series with Euler products. Glasgow mathematical journal, Tome 25 (1984) no. 2, pp. 203-206. doi: 10.1017/S0017089500005620
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author = {Raghavan, S.},
title = {On {Ramanujan} and {Dirichlet} series with {Euler} products},
journal = {Glasgow mathematical journal},
pages = {203--206},
year = {1984},
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doi = {10.1017/S0017089500005620},
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